1. Field
The present disclosure relates generally to strength measurement of materials. More particularly, the invention relates to a non-destructive method and apparatus for measuring the strength of a bond interface between bonded structures.
2. Background
Bonded joints are widely used in a variety of structural applications, and more specifically, primary composite structures are often bonded together in select aerospace applications. The strength of the bonded joint between composite structures needs to be certifiable if the bonded joint is used in a primary structure application. Conventional measurement of bond strength generally involves static proof testing. For the structure to be used in aerospace service, structural proof testing can be performed to the limit load. Limit load is the maximum load that a structure is expected to see in service. However, the cost of such testing is usually high. Physical testing to failure to validate strength is more commonly performed using witness coupons. The witness coupons are samples that are assembled at the same time as the bonded joint using the same materials and following as closely as possible the bonding process of the real structure. The coupons are then tested to failure to validate the process quality. While useful as an indicator, these test articles are not a part of the vehicle structure.
Non-destructive testing methods exist for composite structures; however, the non-destructive methods generally detect the presence of a defect such as a void or unbond with in a joint not the strength thereof. Such methods may include, for example, ultrasonics, x-rays, and acoustics, among others commonly known in the art.
Laser bond inspection is a method of testing bonds using stress waves. The method involves depositing laser energy onto the front surface of a bonded article and generating compression stress waves that reflect off of the back surface of the bonded article as tensile stress waves. The tensile waves predominantly provide the stresses that test the bond between similar materials. If a tensile wave of sufficient strength encounters a weak bond, the bond will separate. A tensile wave is a wave that causes tension in a material. Unfortunately, when the bond is between dissimilar materials the wave reflected from the back surface may not create sufficient tension to evaluate the strength of the bond.
The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings.